The present invention relates to a high-performance bonded magnet useful for wide ranges of magnet applications such as various rotors, magnet rolls for electromagnetic developing-type printers and photocopiers, audio speakers, buzzers, attracting or magnetic field-generating magnets and having a higher residual magnetic flux density Br (or higher residual magnetic flux density Br and coercivity iHc) than those of the conventional Sr and/or Ba ferrite powders, and a ferrite powder used therefor and a method for producing such a bonded magnet and a ferrite powder, more particularly to a magnet roll composed of such a high-performance bonded magnet and a method for producing such a magnet roll.
As well known, bonded magnets are lighter in weight and higher in dimensional accuracy than sintered magnets and suitable for mass production of articles having complicated shapes, and therefore they are widely used for various magnet applications. Recently, magnet-applied products have been drastically miniaturized and reduced in weight, requiring high-performance ferrite bonded magnets having a higher Br (or higher Br and iHc) suitable for miniaturization and reduction in weight.
Conventional Sr and/or Ba ferrite bonded magnets are obtained by bonding Sr and/or Ba ferrite powder having a composition represented by AO.nFe2O3, wherein A is Sr and/or Ba, and n=5-6, with binders such as thermoplastic polyolefin resins or rubbers, advantageous in low cost. However, the ferrite bonded magnets are lower in Br and maximum energy product (BH)max than sintered ferrite magnets to the extent of volume increase due to non-magnetic portions occupied by binders. To obviate this disadvantage, various attempts have been made conventionally to improve the orientation of a ferrite powder by a magnetic field or a mechanical stress applied for a ferrite powder orientation, and to improve the filling of a ferrite powder in binders. As a result, it is almost considered that no further improvement in magnetic properties would be able to be achieved in bonded magnets comprising a ferrite powders having conventional compositions.
If the filling ratio of a ferrite powder in rubbers or plastics is increased to improve the magnetic properties of bonded magnets, the resultant blends would have extremely high melt viscosity. Even though high-melt viscosity blends are subjected to practical orientating magnetic field or mechanical stress, it would be difficult to obtain bonded magnets having well-oriented ferrite powder. This difficulty is remarkable in an injection molding method, though it is appreciable in an extrusion method and a compression molding method, too. Though the orientation of a ferrite powder in the ferrite bonded magnets is improved by increasing the filling ratio of a ferrite powder in rubbers or plastics, such improvement inevitably causes the deterioration of magnetic properties, failing to satisfy the demand of miniaturization and reduction in weight.
To obviate such problems of conventional technology, it is effective to improve the saturation magnetization "sgr"s or crystal magnetic anisotropy constant of a ferrite powder for bonded magnets. Improvement in "sgr"s directly leads to improvement in coercivity Hc (or iHc). Incidentally, the conventional a ferrite powder for bonded magnets having a composition of AO.nFe2O3 has a magnetoplumbite-type crystal structure, and W-type ferrite having larger "sgr"s than a ferrite powder having a magnetoplumbite-type crystal structure has also been investigated. However, the mass production of the W-type ferrite cannot be materialized so far due to difficulty in the control of a sintering atmosphere.
Japanese Patent Laid-Open No. 9-115715 discloses a ferrite powder for bonded magnets having a main phase constituted by a hexagonal magnetoplumbite-type ferrite represented by the general formula: A1xe2x88x92xRx(Fe12xe2x88x92yMy)zO19, wherein A is at least one element selected from the group consisting of Sr, Ba, Ca and Rb, R is at least one of rare earth elements including Y, La being indispensable, M is Zn and/or Cd, and x, y and z are molar ratios meeting the conditions of 0.04xe2x89xa6xxe2x89xa60.45, 0.04xe2x89xa6yxe2x89xa60.45, and 0.7xe2x89xa6zxe2x89xa61.2. Investigation by the inventors has revealed, however, that it is difficult to obtain bonded magnets having high Br and iHc (for instance, exceeding 3.5 kOe) from this a ferrite powder for bonded magnets.
Accordingly, an object of the present invention is to provide a high-performance bonded magnet having a magnetoplumbite-type crystal structure suitable for mass production, which has higher Br (or higher Br and iHc) than those of conventional Sr and/or Ba ferrite bonded magnets, a magnet roll composed of such a bonded magnet, a ferrite powder used for such a bonded magnet, and methods for producing them.
The inventors have paid attention to the fact that by adding metal compounds (for instance, a combination of a La oxide and at least one oxide of Co, Mn, Ni and Zn, or a combination of a rare earth oxide mixture based on a La oxide as a main component and containing oxides of Nd, Pr, Ce, etc. and an oxide of Co and/or Zn), which have not been conventionally tried, to ferrite represented by AO.nFe2O3, wherein A is Sr and/or Ba, and n is 5-6, part of A and Fe elements in the above ferrite can be substituted by the metal elements in the metal compounds added, resulting in a ferrite powder suitable for bonded magnets, which has a magnetoplumbite-type crystal structure with a higher saturation magnetization and coercivity than those of conventional Sr and/or Ba ferrite powder.
The magnetism of this magnetoplumbite-type a ferrite powder is derived from a magnetic moment of Fe ions, with a magnetic structure of a ferri-magnet in which magnetic moment is arranged partially in antiparallel by Fe ion sites. There are two methods to improve the saturation magnetization in this magnetic structure. The first method is to replace the Fe ions at sites corresponding to the antiparallel-oriented magnetic moment with another element, which has a smaller magnetic moment than Fe ions or is non-magnetic. The second method is to replace the Fe ions at sites corresponding to the parallel-oriented magnetic moment with another element having a larger magnetic moment than Fe ions.
Also, increase in a crystal magnetic anisotropy constant in the above magnetic structure can be achieved by replacing Fe ions with another element having a stronger interaction with the crystal lattice. Specifically, Fe ions are replaced with an element in which a magnetic moment derived from an orbital angular momentum remains or is large.
With the above findings in mind, research has been conducted for the purpose of replacing Fe ions with various elements by adding various metal compounds such as metal oxides. As a result, it has been found that Mn, Co and Ni are elements remarkably improving magnetic properties.
However, the mere addition of the above elements would not provide a ferrite powders with fully improved magnetic properties, because the replacement of Fe ions with other elements would destroy the balance of ion valance, resulting in the generation of undesirable phases. To avoid this phenomenon, ion sites of Sr and/or Ba should be replaced with other elements for the purpose of charge compensation. For this purpose, the addition of at least one of La, Nd, Pr, Ce, etc., particularly La, is effective. That is, it has been found that a ferrite powder produced by the addition of an R element compound based on La and an M element compound (at least one of Co, Mn, Ni and Zn) provides bonded magnets having higher Br (or higher Br and iHc) than those of the conventional Sr and/or Ba ferrite bonded magnets. It has also been found that bonded magnets formed by using a ferrite powder produced by the addition of an R element compound based on La and a Co element compound and/or a Zn compound has well-balanced Br and iHc, particularly suitable for magnet rolls.
Further investigation of the inventors has revealed that sufficient improvement in the magnetic properties of bonded magnets cannot be obtained only by selecting the composition of main components for ferrite. This is because the magnetic properties of bonded magnets containing a ferrite powder are largely affected not only by the basic composition of a ferrite powder but also by the amounts of impurities (particularly Si, Ca, Al, Cr) in the ferrite powder.
In general, when magnetically isotropic ferrite obtained by a ferritization reaction is pulverized to fine particles having particle sizes substantially corresponding to the single magnetic domain size and then heat-treated, the resultant a ferrite powder for bonded magnets has improved magnetic properties. Investigation of the inventors has also revealed that to achieve as high Br as corresponding to a Br potential inherent in a ferrite powder materials adjusted to have the above basic composition, the amounts of additives for forming crystal grain boundary phases such as SiO2, CaO, etc., useful for sintered a ferrite powders, and Al2O3 and/or Cr2O3 having a function to largely improve iHc though remarkably decreasing Br should be made as small as possible.
The first factor affecting the amounts of inevitable impurities in ferrite powder is the purity of iron oxide. Iron oxide, which is a main component for the ferrite powder, inevitably contains inevitable impurities such as SiO2, Al2O3, Cr2O3, etc. Though the amounts of these inevitable impurities are preferably as small as possible, the used of iron oxide having higher purity than necessary for industrial production disadvantageously leads to increase in production cost. Incidentally, other starting materials than iron oxide are preferably SrCO3, La2O3, Co oxides, etc. having a purity of 99% or more.
The second factor affecting the amounts of inevitable impurities in ferrite powder is Si, Cr, Al, etc., which may enter into the ferrite powder in the course of fine pulverization of a magnetically anisotropic ferrite composition obtained by a ferritization reaction to a single magnetic domain size or a particle size corresponding thereto. As a result of investigation by the inventors, it has been appreciated that the amounts of inevitable impurities tend to increase in the case of using a ball-milling pot or balls made of steel, which are in general widely used in the production of ferrite powder. It has been found that particularly when fine pulverization is carried out to an average diameter of about 1.3 xcexcm or less measured by an air permeation method using a Fischer Subsieve sizer, portions in contact with the ferrite powder, such as steel balls (pulverization medium), inner walls of pulverizing chambers, etc., are extremely worn, resulting in Si, Cr and Al components entering into the ferrite powder. The extent of contamination is remarkable when the average diameter of pulverized powder is as small as 1.1 xcexcm or less.
From the aspect of commercial production, it is preferable to use usual pulverizing machines such as attritors, ball mills, vibration ball mills, etc., and also steel balls that less affect the magnetic properties of ferrite powder than ceramic balls when worn pulverization media contaminate the ferrite powder. Accordingly, it has been found to be necessary to select a type of steel that does not substantially contain Al as a material for the inner walls of pulverization chambers, pulverization media, etc., to prevent Al and other inevitable impurities from entering into the ferrite powder during a pulverization process.
However, because the inclusion of Si and Cr components into the ferrite powder during pulverization is unavoidable due to limitations in commercial production, the permissible amounts of Si, etc. contained in the ferrite powder and the amounts of Si, etc. entering into the ferrite powder during pulverization have been taken into consideration to achieve the following findings: When iron oxide used for ferrite powder for bonded magnets has a total content of Si and Ca calculated as (SiO2+CaO) is 0.06 weight % or less and a total content of Al and Cr calculated as (Al2O3+Cr2O3) is 0.1 weight % or less, the resultant ferrite powder contains impurities in such amounts that a total of a Si content calculated as SiO2 and a Ca content calculated as CaO is 0.2 weight % or less, and a total of an Al content Al2O3 and a Cr content calculated as Cr2O3 is 0.13 weight % or less, resulting in higher Br than that of the conventional ferrite powder. The present invention has been completed based on this finding.
Thus, the ferrite powder for bonded magnets according to the present invention has a substantially magnetoplumbite-type crystal structure and an average diameter of 0.9-2 xcexcm, the ferrite powder having a basic composition represented by the following general formula:
(A1xe2x88x92xRx)O.n[(Fe1xe2x88x92yMy)2O3] by atomic ratio, wherein A is Sr and/or Ba; R is at least one of rare earth elements including Y, La being indispensable; M is at least one element selected from the group consisting of Co, Mn, Ni and Zn; and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)], and
5xe2x89xa6nxe2x89xa66,
a total of an Si content (calculated as SiO2) and a Ca content (calculated as CaO) being 0.2 weight % or less, and a total of an Al content (calculated as Al2O3) and a Cr content (calculated as Cr2O3) being 0.13 weight % or less.
The method for producing a ferrite powder for bonded magnets according to the present invention comprises the steps of preparing a magnetically isotropic ferrite composition having the above basic composition; finely pulverizing the ferrite composition; and heat-treating the pulverized ferrite powder at 750-950xc2x0 C. for 0.5-3 hours in the air.
It is preferred that the magnetically isotropic ferrite composition is prepared by mixing an iron oxide with a compound containing an A element, a compound containing an R element and a compound containing an M element and then calcining the resultant mixture for a solid-state reaction, and that the magnetically isotropic ferrite composition is subjected to dry-pulverization to an average diameter, heat treatment, immersion in water for disintegration, and then drying. Further, iron oxide obtained by spray-roasting a waste liquid generated by washing steel with hydrochloric acid is preferably used as the iron oxide.
The total of a Si content (calculated as SiO2) and a Ca content (calculated as CaO) is preferably 0.15 weight % or less, and the total of an Al content (calculated as Al2O3) and a Cr content (calculated as Cr2O3) is preferably 0.1 weight % or less in the ferrite powder.
The anisotropic granulated powder for bonded magnets constituted by an aggregate of ferrite powder having a substantially magnetoplumbite-type crystal structure and an average diameter of 0.9-2 xcexcm, said ferrite powder having a basic composition represented by the following general formula:
(A1xe2x88x92xRx)Oxc2x7n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba; R is at least one of rare earth elements including Y, La being indispensable; M is at least one element selected from the group consisting of Co, Mn, Ni and Zn; and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)], and
5xe2x89xa6nxe2x89xa66,
the anisotropic granulated powder having an average diameter of more than 2 xcexcm and 10 xcexcm or less, and a total of an Si content (calculated as SiO2) and a Ca content (calculated as CaO) being 0.2 weight % or less, and a total of an Al content (calculated as Al2O3) and a Cr content (calculated as Cr2O3) being 0.13 weight % or less.
The method for producing an anisotropic granulated powder for bonded magnets according to the present invention comprises the steps of calcining a starting material mixture having the above basic composition for ferritization to form magnetically isotropic ferrite, which is pulverized, molded in a magnetic field, disintegrated to an average diameter of more than 2 xcexcm and 10 xcexcm or less, and then heat-treated at 750-950xc2x0 C. for 0.5-3 hours in the air.
The bonded magnet according to the present invention comprises the above ferrite powder or the above anisotropic granulated powder and a binder, having radial or polar anisotropy.
The magnet roll according to the present invention has a plurality of magnetic poles on a surface thereof, at least one magnetic pole portion thereof being constituted by a bonded magnet composed of 85-95 weight % of ferrite powder and 15-5 weight % of a binder, the ferrite powder having a substantially magnetoplumbite-type crystal structure, an average diameter of 0.9-2 xcexcm, and a basic composition represented by the following general formula:
(A1xe2x88x92xRx)Oxc2x7n[(Fe1xe2x88x92yMy)2O3] by atomic ratio,
wherein A is Sr and/or Ba; R is at least one of rare earth elements including Y, La being indispensable; M is at least one element selected from the group consisting of Co and/or Zn; and x, y and n are numbers meeting the following conditions:
0.01xe2x89xa6xxe2x89xa60.4,
[x/(2.6n)]xe2x89xa6yxe2x89xa6[x/(1.6n)], and
5xe2x89xa6nxe2x89xa66,
a total of an Si content (calculated as SiO2) and a Ca content (calculated as CaO) being 0.2 weight % or less, and the total of an Al content (calculated as Al2O3) and a Cr content (calculated as Cr2O3) being 0.13 weight % or less in the ferrite powder.